1. Field of the Invention
The present invention relates to an optical device which detects the focus state of an object to perform automatic focus adjustment operation.
2. Related Background Art
As one type of conventional focus detection means for a camera, a means which determines an in-focus state by observing a correlative position displacement between two images formed by light beams that have passed through pupil areas obtained by splitting the exit pupil of a photo-taking lens is known. For example, a secondary imaging system which guides optical images formed on a prospective focal plane (a plane corresponding to a film surface) to two sensor surfaces by two juxtaposed secondary imaging optical systems, and detects the correlative position displacement between these two images is disclosed in Japanese Patent Application Laid-Open Nos. 55-118019, 55-155331, and the like.
In such focus detection means, if it is determined that the two obtained images have poor reliability (low contrast), focus detection incapable operation is done. As the focus detection incapable operation, so-called "search operation" for performing focus detection by moving the photo-taking lens by a predetermined amount or while continuously moving the photo-taking lens, to expect an increase in contrast, is disclosed in Japanese Patent Publication No. 6-5335, or the like.
On the other hand, a so-called "multi-point AF camera" which has a plurality of focus detection means to detect the focus states of a plurality of areas in the frame, and adjusts the focus of the photo-taking lens on the basis of the detection results is known. The plurality of areas that allow focus detection in the frame are also called "focus detection points" Furthermore, a camera having a so-called line of sight detection device for detecting a position in the viewfinder frame where the photographer is gazing has also been proposed.
For example, Japanese Patent Application Laid-Open No. 1-241511 discloses a camera which senses an image of a front eye portion of an eyeball of the photographer illuminated with light emitted by an infrared emitting diode (to be abbreviated as IRED hereinafter) using an area sensor, detects the line of sight coordinate position of the photographer on the viewfinder by processing the obtained image signal, and selects one of a plurality of focus detection points or photometry areas of the multi-point AF camera on the basis of the detection result.
An auto focus mode of a conventional camera includes two modes, i.e., a one-shot AF mode in which focus detection is continued until the photo-taking lens reaches an in-focus state, and is stopped once the in-focus state is reached, and a servo AF mode in which focus detection is continued irrespective of the focus state of the photo-taking lens.
The operation in the one-shot AF mode of the camera with the aforementioned line of sight detection means is as follows.
When a switch SW1 is turned on at the first stroke position of a release button, the line of sight detection means detects the gazing point of the photographer in the viewfinder, and determines a focus detection point corresponding to the detected gazing point. Then, the focus detection means detects the focus state at the focus detection point determined by the line of slight detection means, and the photo-taking lens is driven to an in-focus position on the basis of that information. Once the line of sight detection means determines the focus detection point, focusing is done in consideration of only the focus state of that focus detection point until an in-focus state is reached.
In the one-shot AF mode, since the release switch cannot be pressed unless an in-focus state is reached, no line of sight detection is done during release operation (including continuous shot operation, of course).
The operation in the servo AF mode of the camera with the line of sight detection means is as follows.
As in the one-shot AF mode, immediately after the switch SW1 is turned on, the line of sight detection means operates to determine the focus detection point. After that, the photo-taking lens is driven to an in-focus point on the basis of focus detection calculation information at that focus detection point. In this mode, while the switch SW1 is ON, detection and calculation of the focus state, and lens driving repeat themselves.
In the servo AF mode as well, no line of sight detection is done during release operation (including continuous shot operation, of course) as in the one-shot AF mode.
In such camera, the following problems are posed upon selecting the servo AF mode. The problems will be explained below with reference to FIGS. 6A and 6B.
FIGS. 6A and 6B show in-finder views. Referring to FIG. 6A, the focus states of the photo-taking lens are detected at focus detection points 201 to 219. Such multi-point AF can be implemented by using Japanese Patent Application Laid-Open No. 9-184968. Note that these focus detection points 201 to 219 are indicated in a transmission LCD panel as a known technique, and FIG. 6A illustrates a state wherein all the focus detection points are indicated. However, when all the focus detection points are indicated all the time during photo-taking, they disturb the field of view of the photographer, and an object is hard to confirm. For this reason, normally, all the focus detection points are OFF and only a focus detection frame 220 printed on a focusing screen is indicated. When the line of sight detection means selects one focus detection point, the selected focus detection point alone is turned on.
In the aforementioned arrangement, a sequence upon servo AF photo-taking will be explained.
The photographer brings a principal object to be taken into the focus detection frame 220, and turns on the switch SW1 while gazing at this principal object. Immediately after the switch SW1 is ON, photometry operation and line of sight detection are executed in turn, thus selecting a focus detection point. When a focus detection point is selected, the selected focus detection point is indicated within the viewfinder. After that, the focus state of that focus detection point is detected, the driving amount of the photo-taking lens is calculated, and the photo-taking lens is driven to a predetermined position.
However, line of sight detection always suffers a detection error. Especially, when the user wears spectacles, the error is large, and the focal detection point of a portion where the principal object is present is not always selected. For example, as shown in FIG. 6B, the focus detection point 219 (see FIG. 6A) where the principal object is present should be selected, but the focus detection point 215 where no principal object is present is selected due to a line of sight detection error.
As shown in FIG. 6A, there is a gap between the neighboring focus detection points, and it is technically very hard to increase the number of focus detection points to fill such gap. For this reason, the principal object may fall between neighboring focus detection points. Even in such case, if the focus detection points are indicated all the time, the photographer can turn on the switch SW1 while adjusting the indication to the principal object, thus preventing the principal object from falling between neighboring focus detection points. However, as described above, since the focus detection points are not indicated all the time, the gazing point may be present in a gap between neighboring focus detection points. At this time, a focus detection point closest to the gazing point is selected against the photographer's will. Especially, when the photo-taking magnification is small, such phenomenon readily occurs.
In either case (line of sight detection suffers an error, and the principal object falls between neighboring focus detection points), since the principal object is not present at the selected focus detection point, focus detection calculation is done on the basis of the background, and as a result, the photo-taking lens is driven to the background. However, even when the lens is driven to adjust the focus on the background, the principal object can often be confirmed within the viewfinder. Hence, when the photographer adjusts the position of the principal object in the viewfinder to the selected focus detection point, the focus can be adjusted on the principal object.
However, when the lens is driven to adjust the focus on the background, the focus cannot always be adjusted on the principal object. For example, in photo-taking at night, if the background is dark, an in-focus incapable state readily occurs. In such in-focus incapable state, the aforementioned search operation is done. However, in such photo-taking environment, the contrast can be hardly raised to enable focus detection during the search operation. In this search operation, the photo-taking lens is driven from the nearest distance side to infinity, and in particular, when the photo-taking lens is driven to the nearest distance side, the principal object is considerably out-of-focus. As a result, the photographer may lose sight of the principal object, and miss an important shutter chance.